Issue #3471134 - Add api file

Closes #3471134

fapi_validation.api.php

+<?php
+
+/**
+ * @file
+ * Describe hooks provided by the Form API Validation module.
+ */
+
+/**
+ * Provides a list of filters.
+ *
+ * @return
+ *   An array of filters and their callbacks.
+ */
+function hook_fapi_validation_filters() {
+  return array(
+    'custom' => array(
+      'callback' => 'custom_filter_callback',
+    ),
+  );
+}
+
+/**
+ * Provides a list of rules.
+ *
+ * @return
+ *   An array of rules and their callbacks.
+ */
+function hook_fapi_validation_rules() {
+  return array(
+    'custom' => array(
+      'callback' => 'custom_rule_callback',
+      'error_msg' => '%field value is invalid.',
+    ),
+  );
+}